Heat treatment after welding of DP980 steel: effect of heat treatment parameters on microstructure and mechanical properties of resistance spot welding

Effect of in-situ Post Weld Heat Treatment (PWHT) parameters on microstructure and mechanical properties of resistance spot welded DP980 steel was investigated. Samples were welded at 8.5 kA and 4 levels of PWHT current (3.5 to 5 kA, with 0.5 kA steps) at a constant PWHT time (80 cycles) and 3 levels of PWHT time (10, 50 and 99 cycles) at a constant PWHT current (4 kA). Results showed that while no noticeable martensite tempering occurred at 3.5 kA PWHT current, PWHT at 4 kA resulted in a fully tempered microstructure consisting mostly of a ferritic matrix with dispersed carbide particles. However, higher PWHT currents increased heat input and consequently temperature at the welded region up to the austenite formation temperature, resulting in formation of new martensite phase upon cooling. Tensile test results showed that at PWHT current of 4 kA, the joint has the highest peak load (load bearing capacity) and fracture energy (13.3 kN and 15.1 J, respectively). For PWHT time, both peak load and fracture energy increased with PWHT time (up to 14.3 kN and 15.7 J, respectively) and microstructure consisted mainly of ferrite and carbide particles. To summarize, PWHT enhanced mechanical properties and reduced martensite phase in microstructure. However, PWHT did not change the fracture mode and interfacial fracture occurred in the as-welded and all PWHT samples.

DP Steel, Post Weld Heat Treatment, Resistance Spot Welding, Martensite Tempering, Interfacial Fracture.